This earlier application discloses a horizontal tree having a comparatively large vertical bore without any vertical in-line internal valves, and which is at least large enough for the tubing hanger to be landed in the bore. A lateral production fluid outlet port leads from the vertical throughbore. The tubing hanger and horizontal tree are provided within complementary guide means to orientate the tubing hanger to a predetermined angular position at which a lateral production fluid outlet port in the tubing hanger is in alignment with that in the horizontal tree.
With such a horizontal tree, it is necessary to remove the blow out preventer (BOP) from the wellhead assembly to install the horizontal tree, and then replace the BOP on the horizontal tree before the tubing completion can be run. When the tubing completion is run, and before the BOP is removed, the production bore must be sealed with a wireline set plug. In addition, for safety reasons, it is necessary to be able to isolate any potential production fluid outlet. Thus, in operation, the wellhead assembly requires at least one wireline set plug in the vertical production bore above the lateral production bore, and also requires at least one gate valve for the production fluid in the lateral production bore.